Die Embryonalentwicklung der Paradiesschnecke ... - TOBIAS-lib

Kapitel 4
arrangement of the snails’ nervous system, including streptoneury, without
the need for a horizontal rotation.
Previous studies have shown that parts of the nervous system that the
visceral loop consists of, may develop before, during, or after torsion. Additionally,
in Pt 2+ -exposed M. cornuarietis we find streptoneury in individuals
whose visceropallium did not undergo any horizontal rotation at all. All these
facts raise the question how the structure of the nervous system in gastropods
is determined. Does the crossing of the pleurovisceral connectives necessarily
have to be caused passively by a movement of their surrounding tissues?
Or could the growth of the pleurovisceral connectives rather be directed
by chemical signals independent from mechanical constraints? Haszprunar
(1988) speculated that cytochemical markers might lead the growth of the
pleurovisceral nerve connectives to their destinations although, in his opinion
at that time, this mechanism would not influence the dependence of torsion
and streptoneury. Indeed, chemical components, nerve growth factors, can
influence the survival of neurons or induce and inhibit neurite outgrowth
(Berg, 1984). Hermann et al. (2000) found a homolog of an epidermal growth
factor in Lymnaea stagnalis that can induce neurite outgrowth in vitro and
that may also play a role during the development of the nervous system.
Another group of molecules that may act as morphogens and regulators of
neurogenesis are neurotransmitters (Buznikov et al., 1996). In isolated Helisoma
nerve cells, serotonin can inhibit neurite outgrowth (Haydon et al.,
1987) and changing the serotonin content in developing Helisoma embryos
leads to alterations in the development of specific neurons (Goldberg and
Kater, 1989).
Croll and Voronezhskaya (1996) investigated neurogenesis in Lymnaea and
observed very early appearing nerve cells, which are positioned in the posterior
region of the body and that seem to serve as a scaffold for the developing
ganglia and connectives. Dickinson et al. (1999; 2000) also observed very
early developing nervous structures that may serve as “guidances” for the
developing adult central nervous system in Crepidula fornicata (1999) and
Aplysia californica (2000). In gastropods in which the pleurovisceral nerve
connectives develop only after torsion, such nerve structures, which are al-
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